Both estrogen and prolactin are essential hormones for the maintenance and growth of mammary carcinomas induced by dimethylbenzanthracene. (DMBA-tumor). In this proposal, the investigator plans to elucidate the roles of these two hormones and their interrrelationship in promoting tumor growth. It is assumed, from preliminary experimental results, that a feed-back mechanism between prolactin and estrogen exists in situ in the tumor cells. Such mechanism is related to the influence of estrogen upon prolactin receptors, and vice versa, the efffects of prolactin upon estrogen receptors. Specific attention to the influence of other hormones, either as antagonists or synergists of these two hormones, upon the growth of tumors will be compared to their effects upon the levels of prolactin receptors and estrogen receptors. Prolactin-induced and estrogen-induced proteins as end-markers will be determined to relate to the changes in the levels of these receptors and to tumor growth. In order to clearly define the interrelationship of different hormones to this estrogen-prolactin mechanism, besides in vivo experiments performed with the DMBA-tumor model, several hormonal dependent or responsive cell lines, from both human and experimental animals will be utilized for these studies. This investigation is aimed to provide pertinent insight regarding the biochemical meaning of synergism and antagonism in hormonal interaction.